Clarification of bayer process liquors

ABSTRACT

The clarification of a sodium aluminate liquor in the Bayer Process, especially the liquor obtained by Kelly filter filtration of the supernatant from the primary settling stage, is improved by flocculation with a polymer of the formula CH 2  ═C(R 1 )CONHR 2  NR 3  R 4  wherein R 1  is hydrogen or methyl, R 2  is straight or branched chain C 2-8  alkylene having at least 2 carbon atoms in the backbone and R 3  and R 4  are independently selected from C 1-4  alkyl.

The separation of inorganic suspended solids from an aqueous liquor isoften promoted by the addition to the liquor of a polymeric flocculantthat may be non-ionic, anionic or cationic.

When the liquor is alkaline, the flocculant is usually an anionic ornon-ionic polymer but there have been some proposals to use a cationicpolymer in alkaline liquors.

In U.S. Pat. No. 3,023,162, partially or wholly quaterniseddialkylaminoalkyl (meth) acrylates are used for the flocculation ofaqueous suspensions having pH from 9 to 14 and in which the suspendedsolids are organic (e.g., sewage) or inorganic (e.g., mineral dressingliquors).

In U.S. Pat. No. 4,160,731, a copolymer of acrylamide and3(methacrylamido)-propyl-trimethyl ammonium chloride (MAPTAC) is used topromote dewatering of sewage sludge to which sufficient lime has beenadded to raise the pH to at least 12.

Particular problems arise in the clarification of Bayer Process liquors.The Bayer process is a widely used process for obtaining aluminium frombauxite by digesting the bauxite with sodium hydroxide to form a slurryof red mud, separating the red mud solids from the liquor, and thenrecovering the aluminium from the separated liquor. It is important thatthe liquor should be of high purity since impurities are carried intolater stages of the aluminium recovery process and reduce the purity ofthe resultant aluminium and/or make the later recovery stages moredifficult to operate. For instance the presence of inorganic suspendedsolids in the liquor is liable to lead to the contamination of thealuminium by, for instance, iron whilst the presence of dissolvedorganic materials such as humates is liable to interfere with thecrystallisation during the recovery processes.

The separation of inorganic suspended solids is therefore usuallyconducted in two stages, namely a primary settlement stage to removemost of the suspended solids and a pressure filtration stage to clarifyor polish the resultant liquor. Flocculants such as starch or sodiumpolyacrylate are generally used to promote the settlement stage butadditional steps usually have to be taken to improve the effectivenessof the pressure filtration stage, for instance because the suspendedparticles are very fine, and conventionally a filter aid such as acalcium compound is added. Merely increasing the amount of anionic ornon-ionic flocculant to the settler stage does not improve results, andgenerally makes them worse, both because of overdosing in this stage andbecause the extra amount of dissolved polymer is liable to increase theblinding of the filters.

In JP No. 5009646A, it is proposed to promote settlement of the red mudslurry in the primary settlement stage by addition of sodiumpolyacrylate followed by quaternised dialkylaminoalkyl(meth)acrylate.

In U.S. Pat. No. 4,578,255 (not published at the priority date of thisapplication), it is proposed to remove humates from Bayer Processliquors by a particular process involving the addition of a watersoluble, vinyl, polymeric quaternary ammonium salt. The only vinylpolymers which are shown to work satisfactorily are polymers of diallyldimethyl ammonium chloride (DADMAC) and although some polymers havingintrinsic viscosity (IV) up to 4.8 are mentioned, it is clear that bestresults are obtained with DADMAC polymers having IV up to about 1. Novinyl polymers other than DADMAC are mentioned in U.S. Pat. No.4,578,255.

There is therefore still an urgent need to provide better clarificationof sodium aluminate liquors, and in particular to promote betterfiltration since the present methods result in frequent blinding of thefilter cloths thereby reducing the rate of flow through the filters andnecessitating more frequent cleaning off of the cloths.

In the invention, inorganic suspended solids in a sodium aluminateliquor obtained in the Bayer Process are flocculated by addition of aflocculating agent and are separated from the liquor, and the flocculantcomprises a quaternised polymer having intrinsic viscosity above 1 dl/gand which comprises quaternised recurring units derived from monomer ofthe formula

    CH.sub.2 ═C(R.sup.1)CONHR.sup.2 NR.sup.3 R.sup.4

wherein R¹ is hydrogen or methyl, R² is straight or branched chain C₂₋₈alkylene having at least 2 carbon atoms in the backbone and R³ and R⁴are independently selected from C₁₋₄ alkyl. R¹ is preferably methyl. R²may be for instance ethylene, isopropylene, t-butylene or2-ethyl-1,6-hexylene, but is preferably 1,3-propylene. R³ and R⁴ areusually independently selected from methyl and ethyl. The preferredrecurring unit is derived from dimethylaminopropyl methacrylamide(DMAPMA).

Suitable quaternising groups are C₁₋₄ -alkyl or hydroxyalkyl, such asmethyl, ethyl, propyl or 2-hydroxy ethyl, preferably methyl or ethyl.Suitable counterions are chloride, sulphate, methyl sulphate, acetateand nitrate. The recurring unit may for instance be quaternised withethylene oxide or epichlorhydrin in an acid, to give theN-(2-hydroxyethyl) derivatives.

The quaternising groups may themselves contain quaternary nitrogenatoms, for instance as described in U.S. Pat. No. 4,495,367. Preferredquaternising compounds are methyl chloride and dimethyl sulphate.

The polymer may be a homopolymer of the specified units or a copolymerwith other copolymerised ethylenically unsaturated monomers, generallyother acrylic monomers, provided these other monomers do notsignificantly detract from the required properties of the polymer i.e.stability at high pH and temperature. In some instances, thehomopolymers are preferred but copolymers are often more efficientcommercially. Generally the polymer contains at least 10%, andpreferably over 25 or 30%, of the specified quaternised units. Often theamount is over 50% and particularly good results are often achieved withabove 80%, for instance 90 to 95% of the specified units (allpercentages are by weight). Particularly preferred polymers are thoseformed of 95 to 25% by weight of the specified quaternised units and 5to 75% by weight of ethylenically unsaturated comonomer. The comonomersare usually non-ionic and suitable comonomers include (meth)acrylamide,N-vinyl-N-methylacetamide, vinyl pyrollidone and vinyl acetate.Generally the comonomer is acrylamide.

The polymers have a high molecular weight, usually at least 500,000 andgenerally below 30 million. The intrinsic viscosity of the polymer mustbe at least 1 dl/g and is preferably at least 3 dl/g. Although it couldbe up to 30 dl/g, generally it is not above 8 or 10 dl/g. Good resultsare achieved in the range 4.5 to 8 dl/g but values of 3 to 5 dl/g areoften commercially adequate. It is normally preferred for the polymersto have conventional high solubility and linearity but small amounts ofdeliberate cross linking or chain branching may be provided, forinstance as described in our European patent application No. 86302987.2.

The polymers can be made by conventional techniques such as aqueous gelpolymerisation followed by drying and comminution or by reverse phasepolymerisation often followed by azeotropic distillation, either to givea stable suspension of polymer in non-aqueous liquid or to provide drybeads that can then be separated from the non-aqueous liquid. It isoften preferred to make the polymer as a stable suspension in oil ofsmall (e.g., below 4 μm) particles by reverse phase polymerisation,generally followed by azeotropic distillation.

The polymer can be added to the sodium aluminate liquor in conventionalmanner, generally as a dilute aqueous solution formed by dissolvingsolid polymer in water or by mixing into water a reverse phasesuspension of polymer, often in the presence of oil-in-water emulsifier.

The invention can be applied to the separation of inorganic suspendedsolids from any sodium aluminate liquor. In practice this will usuallyhave a pH of above 11 and usually above 13, for instance at least 13.5and often at least 14. A particular advantage of the invention is thatthe process is effective even when the liquor has an elevatedtemperature, for instance above 60° C. and often above 80° C. Typicallythe liquor can have a temperature of at least 95° C. and satisfactoryresults can be obtained at temperatures of up to 115° C. or more.

The sodium aluminate liquor can be any of the caustic sodium aluminateliquors that are obtained in the Bayer Process and from which it isnecessary to separate inorganic suspended solids but preferably it isthe slurry obtained by digesting bauxite with sodium hydroxide,optionally with subsequent dilution. This slurry is then subjected to aprimary separation stage at which red mud is separated from it and issubsquently subjected to one or more washing stages. The liquor leavingthe primary settlement stage normally contains below 0.3% by weightsuspended inorganic solids and is then subjected to a pressurefiltration stage to achieve clarification or polishing of the liquor toreduce the suspended solids content still further. The use of thequaternised polymer of the invention is manifested particularly at thisstage, i.e., it promotes filtration and gives particularly good clarityand low solids content in the filtered liquor.

The invention can be performed by adding the quaternised polymer to theliquor from the primary separation stage, in which event that stage maybe conducted using conventional anionic or non-ionic flocculants, suchas starch or sodium polyacrylate. However good results, and in manyinstances improved results, are obtained if the quaternised polymer isadded before the primary separation stage. Although separation can beachieved using the quaternised polymer as the sole flocculant, it isgenerally desirable to accelerate the separation by using it incombination with a conventional non-ionic or anionic flocculant, bestresults generally being achieved by adding the quaternised polymerbefore adding the anionic or non-ionic flocculant. Suitable flocculantsthat can be used with the quaternised polymer include starch and watersoluble anionic synthetic polymers such as homopolymer or copolymers ofacrylic or methacrylic acid or other ethlenically unsaturated carboxylicor sulphonic acids (generally as the sodium or other water soluble salt)or sulphomethylated acrylamide.

The pressure filtration is generally conducted by a Kelly filter and theresultant liquor is then usually passed to a stage at which aluminahydrate is precipitated from it.

Although the process is of particular value in promoting the rate offiltration and the quality of the filtered sodium aluminate liquor, theprocess is also of use in other stages where adequate separation can beachieved by settlement, without filtration. A particular advantage ofthe invention is that good clarity is obtained without blinding thefilter, and so good filtration rates can be achieved.

The following examples illustrate the invention. In these:

MAPTAC--3-(methacrylamido)propyl trimethyl ammonium chloride.

DADMAC--diallyl dimethyl ammonium chloride

DMAEA.MeClq--dimethyl amino ethyl acrylate quaternised with methylchloride

Acm--acrylamide

All monomer ratios in the following list are by weight. The followingpolymers are tested;

A--MAPTAC homppolymer prepared by gel polymerisation IV 5.1 dl/g(invention).

B--70:30 MAPTAC: Acm copolymer IV 5.0 dl/g (invention).

C--DADMAC polymer IV <1 dl/g.

D--30:70 DMAEA.MeClq:Acm copolymer IV 5.0 dl/g.

E--42:58 DMAEA.MeClq:Acm copolymer IV 8.0 dl/g.

F--DMAEMA.MeClq homopolymer IV <1 dl/g.

G--Sodium acrylate homopolymer IV 13.2 dl/g.

H--Sodium acrylate homopolymer IV 5.0 dl/g.

I--70:30 sodium acrylate:Acm copolymer IV 9.0 dl/g.

J--dimethylamine/epichlorhydrin condensation polymer IV <1 dl/g.

K--95:5 MAPTAC:Acm prepared by reverse phase polymerisation followed byazeotropic distillation. IV 4.7 dl/g (invention).

EXAMPLE 1

The process comditions in a Bayer process for the recovery of aluminawere simulated by making a suspension of 20 g/l of china clay in a 200g/l sodium hydroxide solution and the suspension was settled using 3mg/l of product G. The supernatant remaining after settlement, which hada turbidity of 20 NTU and a suspended solids content well below 0.3% byweight, was collected as 500 ml aliquots in 600 ml tall form beakers.Each of the aliquots was treated with the test flocculant at doses of0.1, 0.25 and 0.5 mg/l (with the last of these doses being applied by atwo stage addition of 0.25 plus 0.25 mg/l) whilst stirring hard. After 2mins of hard stirring, the stirrer speed was reduced and stirringcontinued for 15 minutes during which period the stirring was stopped atintervals of 5, 10 and 15 minutes to allow settlement, and samples ofthe supernatant were used for turbidity measurements.

The results are as follows:

                  TABLE 1                                                         ______________________________________                                                Dose Turbidity (NTU) after stirring for:-                             Product   mg/l   5 mins      10 mins                                                                             15 mins                                    ______________________________________                                        A         0.1    19.8        19.0  19.0                                                 0.25   16.5        16.5  15.0                                                 0.5    6.6         6.2   5.3                                        C         0.1    20.5        20.0  19.0                                                 0.25   19.0        19.0  18.5                                                 0.5    19.0        17.5  17.0                                       G         0.1    19.0        18.5  17.0                                                 0.25   19.0        19.0  19.0                                                 0.5    19.0        16.0  15.0                                       ______________________________________                                    

The low turbidity obtained with polymer A, the MAPTAC homopolymer of IVabout 5, shows the benefit obtained from the invention compared to theuse of a further dosage of polymer G, sodium polyacrylate, or from theuse of the DADMAC polymer, polymer C. This demonstrates thatirrespective of the effect of the DADMAC polymer on the dissolved humatecontent (see U.S. Pat. No. 4,578,255), the process of the inventiongives greatly reduced suspended solids compared to the process of U.S.Pat. No. 4,578,255.

EXAMPLE 2

The process of example 1 was repeated but with stagewise addition ofpolymer doses throughout, i.e. 0.25+0.25+0.5 mg/l, the polymer beingadded as 0.05 g/l solution. The results were as follows:

                  TABLE 2                                                         ______________________________________                                                Dose    Turbidity (NTU) after stirring for:-                          Product   (mg/l)    5 min      10 min                                         ______________________________________                                        B         0.25      17.0       12.0                                                     0.50      8.0        6.4                                                      1.00      5.9        5.2                                            C         0.25      19.0       18.0                                                     0.50      19.0       18.0                                                     1.00      21.0       32.0                                           D         0.25      19.0       18.0                                                     0.50      15.5       15.5                                                     1.00      15.5       15.0                                           E         0.25      18.5       18.0                                                     0.50      15.0       15.0                                                     1.00      14.5       13.5                                           F         0.25      17.5       17.0                                                     0.50      13.0       12.0                                                     1.00      9.2        8.4                                            G         0.25      19.0       19.0                                                     0.50      16.5       16.0                                                     1.00      12.5       12.0                                           H         0.25      19.0       18.0                                                     0.50      16.0       13.5                                                     1.00      15.0       11.5                                           I         0.25      20.0       18.5                                                     0.50      17.0       16.5                                                     1.00      15.5       15.5                                           J         0.25      19.0       17.5                                                     0.50      16.5       15.5                                                     1.00      17.0       16.0                                           ______________________________________                                    

This again demonstrates the effectiveness of the invention (using the70% MAPTAC copolymer B) compared to the use of the DADMAC polymer C orthe other cationic polymers D to F or J, or anionic polymers G, H or I.Although the dimethylaminoethyl methacrylate polymer F gave betterresults than the other cationic polymers, they were inferior to theresults obtained with the MAPTAC polymer B.

EXAMPLE 3

The process of example 2 was repeated except that the liquor wasmaintained at 90° C.

The results were as follows:

                  TABLE 3                                                         ______________________________________                                                Dose    Turbidity (NTU) after stirring for:-                          Product   mg/l      5 mins     10 mins                                        ______________________________________                                        B         0.25      5.8        4.2                                                      0.50      1.8        1.9                                                      1.00      1.1        1.5                                            G         0.25      8.4        8.6                                                      0.50      8.8        9.0                                                      1.00      8.8        8.0                                            ______________________________________                                    

This demonstrates the very good clarification obtained at hightemperatures using the MAPTAC copolymer B compared to the poor resultsusing sodium polyacrylate G.

EXAMPLE 4

The process of example 3 was repeated with 10 minutes total stirring fora range of MAPTAC homopolymers and copolymers of different IV values.The results are shown in Table 4 in which the amount of MAPTAC is givenin percent by weight, the remainder of the polymer being derived fromacrylamide. The turbidity without the polymer addition was 20 NTU.

                  TABLE 4                                                         ______________________________________                                                                      Dose  Turbidity                                 % MAPTAC IV       Floc Size   (mg/l)                                                                              (NTU)                                     ______________________________________                                        100      2.9      small       0.2   3.8                                                         small       0.5   2.9                                       25       10       medium/large                                                                              0.2   2.8                                                         medium/large                                                                              0.5   2.3                                       25       7.5      medium/large                                                                              0.2   2.8                                                         medium/large                                                                              0.5   2.2                                       50       7        large       0.2   2.5                                                         large       0.5   2.1                                       90       4        small       0.2   2.6                                                         small       0.5   3.2                                       100      3.9      small       0.2   2.6                                                         small       0.5   2.6                                       50       7.3      large       0.2   2.3                                                         large       0.5   2.2                                       50       7.4      large       0.2   2.2                                                         large       0.5   2.1                                       25       10.5     large       0.2   2.3                                                         large       0.5   2.05                                      100      1.3      small       0.2   3.2                                                         small       0.5   2.4                                       70       5        medium      0.2   2.0                                                         medium      0.5   1.95                                      ______________________________________                                    

This demonstrates the benefits, from the point of view of floc size andtherefore potential filtration rates, of the use of copolymers of MAPTACwith polyacrylamide.

EXAMPLE 5

A liquor was prepared and settled as in example 1 but using 4 mg/l ofproduct G, various amounts of test polymers were added to thesupernatant and it was then subjected to vacuum filtration. When nopolymer addition was made, the filtration time was 32 seconds, but withthe various MAPTAC copolymers shown in Table 4 and with polymer Ffiltration times were reduced by around 10 second or more with polymerdosages ranging from 0.1 to 1 mg/l.

EXAMPLE 6

A 50% dispersion in oil of a copolymer of 95% MAPTAC and 5% acrylamideand having IV 4.7 was prepared by reverse polymerisation followed byazeotropic distillation. The product was labelled polymer K.

A suspension of 10 g/l china clay in 200 g/l sodium hydroxide was formedand DADMAC polymer C or MAPTAC polymer K was stirred into the suspensionvigorously using a laboratory gang stirrer for 10 minutes. This wasfollowed by 0.5 mg/l product G (sodium polyacrylate). Fast stirring wascontinued for a further minute before reducing to slow stirring for 15minutes. The product was then settled and the turbidity was observed.

In a comparison, product G was added before the settling but products Cand K were added after the settling. The results are shown in Table 5 inwhich turbidity "before" indicates the turbidity obtained when thepolymers C and K are added before settling and turbidity "after" whenthey are added after settling.

                  TABLE 5                                                         ______________________________________                                                          Turbidity (NTU)                                             Product  Dose mg/l      Before   After                                        ______________________________________                                        C        0.25           22.5     26.5                                         C        0.5            13.0     28                                           C        1.0            14.0     32                                           C        2.0            11.0     32                                           K        0.25           16.5     15.5                                         K        0.5            12.0     14.5                                         K        1.0            8.3      15.0                                         K        2.0            6.8      15.0                                         ______________________________________                                    

This clearly demonstrates the benefit of adding the MAPTAC copolymer ofthe invention, and in particular of adding it before the settling.

What is claimed is:
 1. A process for separating inorganic suspendedsolids in a sodium aluminate liquor obtained in a Bayer Process whereinan effective amount of a flocculatng agent is added to the liquorcontaining suspended solids, the flocculating agent comprising aquaternised polymer having an intrinsic viscosity (IV) above 1 dl/g andhaving at least about 25% by weight quaternised recurring units derivedfrom monomers of the formula

    CH.sub.2 ═C(R.sup.1)CONHR.sup.2 NR.sup.3 R.sup.4

wherein R¹ is hydrogen or methyl, R² is straight or branched chain C₂₋₈alkylene having at least 2 carbon atoms in the backbone and R³ and R⁴are independently selected from C₁₋₄ alkyl, and the flocculatedsuspended solids are separated from the liquor.
 2. A process accordingto claim 1 in which the liquor is the slurry obtained by digestingbauxite with sodium hydroxide and the separation is conducted in twostages comprising a primary separation stage and a pressure filtrationstep.
 3. A process according to claim 2 in which the quaternised polymeris added before the primary separation stage.
 4. A process according toclaim 3 in which the flocculant also includes at least one of non-ionicand anionic polymeric flocculant added before the primary stage.
 5. Aprocess according to claim 4 in which the quaternised polymer is addedbefore the non-ionic or anionic polymeric flocculant.
 6. A processaccording to claim 2 in which the quaternised polymer is added after theprimary separation stage and before the filtration stage.
 7. A processaccording to claim 1 or claim 6 in which the quaternised polymer isadded to a sodium aluminate liquor containing below 0.3% suspendedinorganic solids and the liquor is then subjected to pressurefiltration.
 8. A process according to claim 7 in which filtration is bya Kelly filter.
 9. A process according to claim 1 in which R¹ ishydrogen or methyl, R² is 1,3-propylene and R³ and R⁴ are methyl.
 10. Aprocess according to claim 9 in which the quaternerized polymer isderived from 95 to 25% by weight of the quaternized monomer units and 5to 75% by weight of ethylenically unsaturated non-ionic comonomer.
 11. Aprocess according to claim 10 in which the comonomer is acrylamide. 12.A process according to claim 11 in which the polymer has an IV of 3 to10.
 13. A process according to claim 1 in which the polymer isquaternised by methyl chloride.
 14. A process according to claim 1 inwhich the quaternised polymer is derived from 95 to 25% by weight of thequaternised monomer units and 5 to 75% by weight of ethylenicallyunsaturated non-ionic comonomer.
 15. A process according to claim 14 inwhich the comonomer is acrylamide.
 16. A process according to claim 1 inwhich the polymer has IV of 3 to
 10. 17. A process according to claim 1in which the liquor has a temperature of at least 60° C. and pH of atleast
 13. 18. A process for separating inorganic suspended solids from asodium aluminate liquor obtained in a Bayer process wherein an effectiveamount of a flocculating agent is added to the liquor containing thesuspended solids, the flocculating agent comprising a quaternisedpolymer having an intrinsic viscosity of above 1 dl/g and having from 95to 25% by weight quaternised recurring groups derived from monomers ofthe formula

    CH.sub.2 ═C(R.sub.1)CONHCH.sub.2 CH.sub.2 CH.sub.2 N(CH.sub.3).sub.2

wherein R₁ is hydrogen or methyl, and the flocculated suspended solidsare separated from the liquor.
 19. A process according to claim 18 inwhich the polymer is quaternised by methyl chloride.
 20. A processaccording to claim 18 in which the comonomer is acrylamide.
 21. Aprocess according to claim 18 in which the polymer has an intrinsicviscosity of 3 to 10.